You're doing a great job with this. It all meshes well together and creates a great mood.
Add some simple distance based fog. It's a must for any scene. It's also one of the top things people get wrong with cg. It doesn't have to be thick, noticeable, fog. Just some subtle fog in the distance to create the illusion of any atmosphere, and a eye pleasing color fade. It is never not foggy out, and everything fades into the distance.
I'm not sure if your models are just flat shaded or if you don't have any shading other than just flat shadows. Are you toggling smooth shading on your models in blender, and are you using the sharp edge tag + edge shading modifier in Blender? If you don't understand, watch this video tip, it will explain it all. http://cgcookie.com/blender/2010/05/04/tip-using-the-edgesplit-modifier-to-smooth-lowpoly-models/
Your island is flat and looks like a plane instead of a terrain. Subdivide it a bit, and then pull some vertices up or down. If you use the proportional editing tool (the tool the toggles on your menu bar when you press 'o' in edit mode) with smooth or sphere mode, it will be easy to create a nice terrain like shape. It's a small island, so like the fog, use it subtly. This will also help with the flat shading look it currently has.